An energy storage device, especially a lithium secondary battery, has been widely used recently for a power source of a small-sized electronic device, such as a mobile telephone, a notebook personal computer, etc., and a power source for an electric vehicle or electric power storage. Since there is a possibility that such an electronic device or a vehicle is used in a broad temperature range, such as a high temperature in midsummer, a low temperature in an extremely cold region, etc., it is demanded to improve battery characteristics with a good balance in a broad temperature range.
In order to improve an energy density of a lithium secondary battery, alloy-based negative electrode active materials containing silicon, tin, or the like, which has a very large theoretical capacity capable of absorbing and releasing lithium as compared with graphite that is a negative electrode active material most widely used at present, are eagerly studied. However, since when such an alloy-based material absorbs lithium, it causes large volume expansion, the active material is broken and pulverized. Therefore, such an alloy-based material involves such a problem that current collection properties are worsened, or the electrode is liable to expand. In addition, on an active newly formed surface generated due to a crack of the active material, a nonaqueous electrolytic solution is liable to be decomposed. Therefore, there was involved such a problem that decomposed products of the nonaqueous electrolytic solution deposit on the active material surface to cause an increase of resistance or the battery is liable to expand due to the gas generation.
In addition, it is known that on the occasion of using such an alloy-based negative electrode active material for an electrode, when a high-strength resin, such as a polyimide, etc., is used, the cycle property is improved. However, in general, it is necessary to use a polyamic acid (polyimide precursor) dissolved in an organic solvent, such as 1-methyl-2-pyrrolidone, etc., and there was involved such a problem that an equipment for recovering the organic solvent at the time of producing an electrode or for heating to a high temperature of 200° C. or higher to thoroughly achieve imidization has to be introduced, so that an increase in cost is inevitable.
PTL 1 describes a secondary battery having a negative electrode including a carbon powder capable of absorbing and releasing cations, which is integrated with a binder consisting essentially of a polyimide resin.
PTLs 2 to 4 disclose a lithium secondary battery including a negative electrode having an active material layer containing an active material composed of a silicon alloy, an alloy containing tin, or the like and a binder, wherein a polyimide resin whose mechanical properties, such as a tensile elastic modulus, etc., fall within specified ranges is used as the binder. However, PTLs 2 to 4 do not describe a specific chemical structure of the polyimide resin to be used.
PTL 5 discloses a lithium secondary battery including a negative electrode having a negative electrode active material layer formed on a surface of a negative electrode collector, the negative electrode active material layer containing negative electrode active material particles containing silicon and/or silicon alloy particles and a binder; a positive electrode; and a separator, wherein the binder contains a polyimide resin having a specified chemical structure. This polyimide resin is a polyimide having a 3,3′,4,4′-benzophenone tetracarboxylic acid residue.
PTL 6 discloses a nonaqueous electrolytic solution secondary battery, in which a binder of an active material of at least one of a negative electrode and a positive electrode is a polyimide resin, and an elastic modulus of the polyimide resin is 500 to 3,000 MPa, and describes that the cycle property or a capacity storage rate after charging storage is improved.